1. Field of the Invention
The invention relates to an image forming apparatus such as, for example, a copying machine, a printer or a facsimile apparatus which adopts image forming process means of an electrostatic recording type, an electrophotographic recording type or the like, forms an unfixed toner image on a recording medium such as a sheet by a transferring process or a direct process, fixes the unfixed toner image formed on the recording medium as a permanently fixed image on the recording medium by heat-fixing means and outputs the image-formed article.
2. Related Background Art
There has heretofore been proposed an image forming apparatus provided with a plurality of heat-fixing devices as heat-fixing means. There is also an image forming apparatus which tentatively fixes a toner transferred onto a transfer material on the transfer material at a low temperature by the utilization of a plurality of heat-fixing devices differing in fixing temperature from one another, and thereafter regularly fixes the tentatively fixed toner on the transfer material at a high temperature (see Japanese Patent Application Laid-Open No. H03-75772). According to the image forming apparatus adopting such construction, various image conditions such as a line image and image density are not adversely affected, and the toner transferred onto the transfer material can be sufficiently fixed, and an image of high fixing strength and high quality has become obtainable.
However, in a case where the plurality of heat-fixing devices as previously described are disposed, there has been pointed out the problem that the use of the plurality of heat-fixing devices which are originally high in electric power consumption leads to a great increase in consumed electric power in the entire image forming apparatus. Also, a plurality of heat exhaust means for exhausting hot air produced by the heat-fixing devices become necessary so that the interior of the image forming apparatus may not be filled with the hot air. Further, the amount of heat to be exhausted also increases.